Diode-pumped picosecond Ytterbium (Yb)-doped fiber lasers with a repetition rate over 100 MHz that seed a fiber amplifier have attracted interests for their potential uses in the optical frequency metrology. The higher the repetition rate, the more power per mode is delivered in the optical frequency comb. In addition, High-energy picosecond Yb-doped fiber lasers with pulse energies over 1 nJ in the normal-dispersion laser cavity have also been actively researched. Furthermore, the picosecond normal-dispersion Yb-doped fiber lasers have been applied to make fiber-based femtosecond lasers with dechirped pulse-width on the order of 100 fs. Since the output pulses acquired in the normal-dispersion cavity have linear positive chirp, the external compression techniques by using a photonic crystal fiber or a gating pair have been successfully applied.
Environmentally stable operation of the mode-locked fiber lasers by employing polarization maintaining (PM) cavity-components only has been a central issue for the practical long-term operation of the fiber lasers. In the early studies, a Faraday rotator/polarizer pair in the Erbium (Er)-doped fiber laser or an acousto-optic modulator/grating pair for frequency shifted feedback in the Yb-doped fiber laser has been used to make stable single-polarization fiber lasers. Recently, a single-polarization oscillation in a sigma-type cavity was realized by incorporating a non-PM Yb-doped fiber and a hollow core photonic crystal fiber for the stretched-pulse operation.
However, environmentally stable PM Yb-doped fiber lasers in the normal-dispersion regime have only recently been demonstrated with relatively low repetition rates either in a linear standing-wave cavity or a ring traveling-wave cavity. This is partly due to the difficulties to implement the nonlinear polarization rotation (NPR) technique that is widely used in the non-PM fiber lasers. Thus, self-starting mode-locking in the Yb-doped fiber laser has usually been achieved with a semiconductor saturable absorber mirror (SAM). The NPR technique was implemented in a linear cavity by using a Faraday mirror/PM coupler pair. An environmentally stable all-normal-dispersion PM Yb-doped fiber laser with a repetition rate over 100 MHz and a pulse energy over 1 nJ was not reported yet.